Pore pressures and fracture pressures in oil and gas wells vary with depth. The pore pressure at a particular depth is defined to be the pressure exerted by the fluid in the formation at that depth into a well's borehole. Formation fluids will escape into the borehole if the pressure exerted by drilling fluids in the well's borehole is less than the pore pressure. The fracture pressure at a particular depth is the pressure of the drilling fluids in the borehole that can fracture the formation at that depth.
An oil well being drilled is considered underbalanced if the pressure exerted by the drilling fluids is slightly less than the pore pressure. Drilling an underbalanced well is challenging when the well passes through a number of formations having different lithologies.